A file server is a computer that provides file service relating to the organization of information on storage devices, such as disks. The file server or filer includes a storage operating system that implements a file system to logically organize the information as a hierarchical structure of directories and files on the disks. Each “on-disk” file may be implemented as a set of data structures, e.g., disk blocks, configured to store information. A directory, on the other hand, may be implemented as a specially formatted file in which information about other files and directories are stored.
A filer may be further configured to operate according to a client/server model of information delivery to thereby allow many clients to access files stored on a server, e.g., the filer. In this model, the client may comprise an application, such as a database application, executing on a computer that “connects” to the filer over a direct connection or computer network, such as a point-to-point link, shared local area network (LAN), wide area network (WAN), or virtual private network (VPN) implemented over a public network such as the Internet. Each client may request the services of the file system on the filer by issuing file system protocol messages (in the form of packets) to the filer over the network.
A common type of file system is a “write in-place” file system, an example of which is the conventional Berkeley fast file system. By “file system” it is meant generally a structuring of data and metadata on a storage device, such as disks, which permits reading/writing of data on those disks. In a write in-place file system, the locations of the data structures, such as inodes and data blocks, on disk are typically fixed. An inode is a data structure used to store information, such as metadata, about a file, whereas the data blocks are structures used to store the actual data for the file. The information contained in an inode may include, e.g., ownership of the file, access permission for the file, size of the file, file type and references to locations on disk of the data blocks for the file. The references to the locations of the file data are provided by pointers in the inode, which may further reference indirect blocks that, in turn, reference the data blocks, depending upon the quantity of data in the file. Changes to the inodes and data blocks are made “in-place” in accordance with the write in-place file system. If an update to a file extends the quantity of data for the file, an additional data block is allocated and the appropriate inode is updated to reference that data block.
Another type of file system is a write-anywhere file system that does not overwrite data on disks. If a data block on disk is retrieved (read) from disk into memory and “dirtied” with new data, the data block is stored (written) to a new location on disk to thereby optimize write performance. A write-anywhere file system may initially assume an optimal layout such that the data is substantially contiguously arranged on disks. The optimal disk layout results in efficient access operations, particularly for sequential read operations, directed to the disks. A particular example of a write-anywhere file system that is configured to operate on a filer is the Write Anywhere File Layout (WAFL™) file system available from Network Appliance, Inc. of Sunnyvale, Calif. The WAFL file system is implemented within a microkernel as part of the overall protocol stack of the filer and associated disk storage. This microkernel is supplied as part of Network Appliance's Data ONTAP™ software, residing on the filer, that processes file-service requests from network-attached clients.
As used herein, the term “storage operating system” generally refers to the computer-executable code operable on a computer that manages data access and may, in the case of a filer, implement file system semantics, such as the Data ONTAP™ storage operating system, implemented as a microkernel, and available from Network Appliance, Inc. of Sunnyvale, Calif., which implements a Write Anywhere File Layout (WAFL™) file system. The storage operating system can also be implemented as an application program operating over a general-purpose operating system, such as UNIX® or Windows NT®, or as a general-purpose operating system with configurable functionality, which is configured for storage applications as described herein.
Disk storage is typically implemented as one or more storage “volumes” that comprise physical storage disks, defining an overall logical arrangement of storage space. Currently available filer implementations can serve a large number of discrete volumes (150 or more, for example). Each volume is associated with its own file system and, for purposes hereof, volume and file system shall generally be used synonymously. The disks within a volume are typically organized as one or more groups of Redundant Array of Independent (or Inexpensive) Disks (RAID). RAID implementations enhance the reliability/integrity of data storage through the redundant writing of data “stripes” across a given number of physical disks in the RAID group, and the appropriate caching of parity information with respect to the striped data. In the example of a WAFL file system, a RAID 4 implementation is advantageously employed. This implementation specifically entails the striping of data across a group of disks, and separate parity caching within a selected disk of the RAID group. As described herein, a volume typically comprises at least one data disk and one associated parity disk (or possibly data/parity partitions in a single disk) arranged according to a RAID 4, or equivalent high-reliability, implementation.
In order to improve reliability and facilitate disaster recovery in the event of a failure of a filer, its associated disks or some portion of the storage infrastructure, it is common to “mirror” or replicate some or all of the underlying data and/or the file system that organizes the data. In one example, a mirror is established and stored at a remote site, making it more likely that recovery is possible in the event of a true disaster that may physically damage the main storage location or it's infrastructure (e.g. a flood, power outage, act of war, etc.). The mirror is updated at regular intervals, typically set by an administrator, in an effort to catch the most recent changes to the file system. One common form of update involves the use of a “snapshot” process in which the active file system at the storage site, consisting of inodes and blocks, is captured and the “snapshot” is transmitted as a whole, over a network (such as the well-known Internet) to the remote storage site. Generally, a snapshot is an image (typically read-only) of a file system at a point in time, which is stored on the same primary storage device as is the active file system and is accessible by users of the active file system. By “active file system” it is meant the file system to which current input/output operations are being directed. The primary storage device, e.g., a set of disks, stores the active file system, while a secondary storage, e.g. a tape drive, may be utilized to store backups of the active file system. Once snapshotted, the active file system is reestablished, leaving the snapshotted version in place for possible disaster recovery. Each time a snapshot occurs, the old active file system becomes the new snapshot, and the new active file system carries on, recording any new changes. A set number of snapshots may be retained depending upon various time-based and other criteria. The snapshotting process is described in further detail in U.S. patent application Ser. No. 09/932,578, entitled INSTANT SNAPSHOT by Blake Lewis et al., which is hereby incorporated by reference as though fully set forth herein. In addition, the native Snapshot™ capabilities of the WAFL file system are further described in TR3002 File System Design for an NFS File Server Appliance by David Hitz et al., published by Network Appliance, Inc., and in commonly owned U.S. Pat. No. 5,819,292 entitled METHOD FOR MAINTAINING CONSISTENT STATES OF A FILE SYSTEM AND FOR CREATING USER-ACCESSIBLE READ-ONLY COPIES OF A FILE SYSTEM by David Hitz et al., which are hereby incorporated by reference.
The complete recopying of the entire file system to a remote (destination) site over a network may be quite inconvenient where the size of the file system is measured in tens or hundreds of gigabytes (even terabytes). This full-backup approach to remote data replication may severely tax the bandwidth of the network and also the processing capabilities of both the destination and source filer. One solution has been to limit the snapshot to only portions of a file system volume that have experienced changes. Hence, FIG. 1 shows a prior art volume-based mirroring where a source file system 100 is connected to a destination storage site 102 (consisting of a server and attached storage—not shown) via a network link 104. The destination 102 receives periodic snapshot updates at some regular interval set by an administrator. These intervals are chosen based upon a variety of criteria including available bandwidth, importance of the data, frequency of changes and overall volume size.
In brief summary, the source creates a pair of time-separated snapshots of the volume. These can be created as part of the commit process in which data is committed to non-volatile memory in the filer or by another mechanism. The “new” snapshot 110 is a recent snapshot of the volume's active file system. The “old” snapshot 112 is an older snapshot of the volume, which should match the image of the file system replicated on the destination mirror. Note, that the file server is free to continue work on new file service requests once the new snapshot 112 is made. The new snapshot acts as a checkpoint of activity up to that time rather than an absolute representation of the then-current volume state. A differencer 120 scans the blocks 122 in the old and new snapshots. In particular, the differencer works in a block-by-block fashion, examining the list of blocks in each snapshot to compare which blocks have been allocated. In the case of a write-anywhere system, the block is not reused as long as a snapshot references it, thus a change in data is written to a new block. Where a change is identified (denoted by a presence or absence of an ‘X’ designating data), a decision process 200, shown in FIG. 2, in the differencer 120 decides whether to transmit the data to the destination 102. The process 200 compares the old and new blocks as follows: (a) Where data is in neither an old nor new block (case 202) as in old/new block pair 130, no data is available to transfer (b) Where data is in the old block, but not the new (case 204) as in old/new block pair 132, such data has already been transferred, (and any new destination snapshot pointers will ignore it), so the new block state is not transmitted. (c) Where data is present in the both the old block and the new block (case 206) as in the old/new block pair 134, no change has occurred and the block data has already been transferred in a previous snapshot. (d) Finally, where the data is not in the old block, but is in the new block (case 208) as in old/new block pair 136, then a changed data block is transferred over the network to become part of the changed volume snapshot set 140 at the destination as a changed block 142. In the exemplary write-anywhere arrangement, the changed blocks are written to new, unused locations in the storage array. Once all changed blocks are written, a base file system information block, that is the root pointer of the new snapshot, is then committed to the destination. The transmitted file system information block is committed, and updates the overall destination file system by pointing to the changed block structure in the destination, and replacing the previous file system information block. The changes are at this point committed as the latest incremental update of the destination volume snapshot. This file system accurately represents the “new” snapshot on the source. In time a new “new” snapshot is created from further incremental changes.
Approaches to volume-based remote mirroring of snapshots are described in detail in commonly owned U.S. patent application Ser. No. 09/127,497, entitled FILE SYSTEM IMAGE TRANSFER by Steven Kleiman, et al. and U.S. patent application Ser. No. 09/426,409, entitled FILE SYSTEM IMAGE TRANSFER BETWEEN DISSIMILAR FILE SYSTEMS by Steven Kleiman, et al., both of which patents are expressly incorporated herein by reference.
This volume-based approach to incremental mirroring from a source to a remote storage destination is effective, but may still be inefficient and time-consuming as it forces an entire volume to be scanned for changes and those changes to be transmitted on a block-by-block basis. In other words, the scan focuses on blocks without regard to any underlying information about the files, inodes and data structures, which the blocks comprise. The destination is organized as a set of volumes so a direct volume-by-volume mapping is established between source and destination. Again, where a volume may contain a terabyte or more of information, the block-by-block approach to scanning and comparing changes may still involve significant processor overhead and associated processing time. Often, there may have been only minor changes in a sub-block beneath the root inode block being scanned. Since a list of all blocks in the volume is being examined, however, the fact that many groupings of blocks (files, inode structures, etc.) are unchanged is not considered. In addition, the increasingly large size and scope of a full volume make it highly desirable to sub-divide the data being mirrored into sub-groups, because some groups are more likely to undergo frequent changes, it may be desirable to update their replicas more often than other, less-frequently changed groups. In addition, it may be desirable to mingle original and replicated (snapshotted) sub-groups in a single volume and migrate certain key data to remote locations without migrating an entire volume. Accordingly, a more sophisticated approach to scanning and identifying changed blocks may be desirable, as well as a sub-organization for the volume that allows for the mirroring of less-than-an-entire volume.
One such sub-organization of a volume is the well-known qtree. Qtrees, as implemented on an exemplary storage system such as described herein, are subtrees in a volume's file system. One key feature of qtrees is that, given a particular qtree, any file or directory in the system can be quickly tested for membership in that qtree, so they serve as a good way to organize the file system into discrete data sets. The use of qtrees as a source and destination for snapshotted data is desirable.
Where a destination snapshot is updated by a source snapshot, the destination snapshot, which resides on the active file system accessible by users and processes, will remain unstable for a period of time. In other words, if a user or process attempts to read from or write to the snapshot, it would obtain an incomplete picture of the data and directory structure (a destination qtree structure, for example), possible containing incomplete or corrupted branches. It is desirable for this, and other reasons to restrict access to a snapshot while it is being updated.